1. Field of the Invention
The invention relates to an oxide superconducting conductor and a method of manufacturing the same.
2. Description of the Related Art
An RE123-based oxide superconducting conductor is expressed by a composition of RE1Ba2Cu3Oy (RE: a rare earth element, such as Y or Gd, 6.5<y<7.1), has a critical temperature that is higher than the temperature of liquid nitrogen (77 K), and is expected to be applied to superconducting equipment, such as superconducting devices, transformers, fault current limiters, motors, and magnets.
Generally, a superconductor that is formed using an RE123-based oxide superconductor so as to have favorable crystalline orientation exhibits high critical current characteristics in the absence of a magnetic field.
However, when a magnetic field is applied to a superconductor in a superconducting state and an electric current is applied, a Lorentz force is generated in a quantized magnetic flux that has intruded into the superconductor.
At this time, when the quantized magnetic flux is moved by the Lorentz force, a voltage is generated in the direction of the electric current, and a resistance is caused.
Since the Lorentz force increases as the electric current value increases, and the magnetic field becomes stronger, the resistance increases, and the critical current characteristics degrade.
As a solution for the above problem, it is generally known that nanoscale different phases, such as impurities or defects, are doped into a superconducting layer, and the magnetic flux is pinned, thereby improving the critical current characteristics of the superconductor in a magnetic field.
As such a method, for example, a method in which the amount of a superconducting material substituted with Re and Ba, the substrate temperature during formation of a superconducting layer, and the oxygen partial pressure are controlled so as to control the amount of stacking faults caused in the superconducting layer, thereby introducing fine pinning points in a superconducting laminate (refer to Japanese Unexamined Patent Application, First Publication No. 2005-116408) is proposed.
Alternately, a method in which columnar crystals of a Ba oxide having the perovskite structure, such as BaZrO3, BaWO4, BaNb2O6, BaSnO3, BaHfO3, or BaTiO3, are introduced and arranged intermittently in the film thickness direction of a superconducting layer in the superconducting layer composed of an RE123-based oxide superconductor is proposed (refer to Japanese Unexamined Patent Application, First Publication No. 2008-130291).
However, an attempt to improve the critical current characteristics in a magnetic field by introducing stacking defects as described in Japanese Unexamined Patent Application, First Publication No. 2005-116408 becomes a principle in which the optimal conditions for formation of a superconducting thin film, which makes a high critical current possible, are intentionally avoided.
Originally, it is necessary to strictly control the conditions for film formation when a superconducting layer is formed.
Therefore, the method in which a superconducting layer is formed by changing the conditions of film formation, such as the oxygen partial pressure and the substrate temperature, as described in Japanese Unexamined Patent Application, First Publication No. 2005-116408 becomes a film-forming method outside the optimal conditions for film formation.
As a result, there are problems in that the critical current characteristics are significantly deteriorated, it becomes difficult to control the amount of defects, and it becomes difficult to manufacture a superconducting wire having uniform critical current characteristics throughout the longitudinal direction thereof.
In addition, in an attempt to improve the critical current characteristics in a magnetic field by introducing columnar crystals into a superconducting layer as described in Japanese Unexamined Patent Application, First Publication No. 2008-130291, it is easy to control the stacking defects, but there is a problem in that the critical current characteristics are degraded in the absence of a magnetic field or in an extremely low magnetic field area.
In addition, since the columnar crystals grow excessively in parallel with the c axis (the film thickness direction of the superconducting layer; the vertical direction to a substrate of the superconducting wire), a strong effect of suppressing the movement of a quantized magnetic flux can be obtained in a case in which a magnetic field is applied in the c-axis direction; however, in a case in which a magnetic field is applied at other angles, such as in the 45° direction with respect to the c axis, the effect of suppressing the movement of the quantized magnetic flux is weak, and thus there is a problem in that the critical current characteristics become somewhat significantly degraded.